I am the Almighty God: Heb. שַׁדַּי - I am He Whose Godliness suffices for every creature. [שֶׁ that, דַּי is sufficient]. Therefore, walk before Me, and I will be your God and your Protector, and wherever it (this name) appears in Scripture, it means “His sufficiency,” but each one is [to be interpreted] according to the context. — [from Gen. Rabbah 47:3]

walk before Me: As the Targum renders: “Serve Me, cleave to My service.”

התהלך לפני: כתרגומו פלח קדמי, הדבק בעבודתי:

and be perfect: This too is one command following another command: be perfect in all My trials (Mid. Ps. 119:3), i.e., “Walk before Me” with faith and honesty, and also be perfect in all My trials. [Mizrachi] According to its midrashic interpretation, walk before Me refers to the commandment of circumcision, and thereby, you will be perfect, for as long as the foreskin is upon you, I consider you imperfect (Gen. Rabbah 46:1). Another explanation: “and be perfect” - Now you are missing [control over] five organs: two eyes, two ears, and the male organ. I will add a letter to your name, and the numerical value of your letters [of your name] will be 248, corresponding to the number of your organs (Tan. Lech Lecha 16, Ned. 32b).

And Abram fell upon his face: from fear of the Shechinah, for as long as he was uncircumcised, he did not have the strength to stand when the Divine Presence stood over him, and that is what is said concerning Balaam (Num. 24:4):“who falls and his eyes are open” (Num. Rabbah 12:8). I found this in the Baraitha of Rabbi Eliezer (Pirkei d’Rabbi Eliezer ch. 29).

the father of a multitude of nations: אַב הֲמוֹן is an acrostic of his name [i.e., - אב ר הם]. (Gen. Rabbah 46:7). The “resh” that was in it [his name] originally, denoting that he was the father only of Aram, which was his native place, whereas now [he became] the father of the whole world (Ber. 13a): nevertheless the “resh” that was there originally was not moved from its place. For even the “yud” in Sarai’s name complained to the Shechinah until it was added to Joshua, as it is said: (Num. 13:16):“and Moses called Hosea [הוֹשֵׁעַ] the son of Nun, Joshua [יְהוֹשֻׁעַ].” - [from Gen. Rabbah 47:1]

And you: Heb. וְאַתָּה. This “vav” connects [this verse] to the preceding matter. “As for Me, behold My covenant is with you,” and you must be careful to observe it. Now what does its observance entail?“This is My covenant, which you shall observe…that every male among you be circumcised.”

And you shall circumcise: וּנְמַלְתֶּם is like וּמַלְתֶּם, and the “nun” is superfluous, a radical that sometimes appears in it, like the“nun” of נוֹשֵׁךְ and the “nun” of נוֹשֵׂא. וּנְמַלְתֶּם has the same form as וּנְשָׂאתֶם, (i.e., the Kal form). But יִמּוֹל is in the passive form (the Nifal), like יֵעָשֶׂה (it will be done), יֵאָכֵל (it will be eaten).

12And at the age of eight days, every male shall be circumcised to you throughout your generations, one that is born in the house, or one that is purchased with money, from any foreigner, who is not of your seed.

Those born in the house… shall be circumcised: Here Scripture repeated it [the commandment to circumcise a slave born in the house;] but did not state [that it is to be] on the eighth day, to teach you that there is a slave born in the house who is circumcised after eight days [other editions: at the age of one day], as is delineated in Tractate Shabbath (135b).

And an uncircumcised male: Here Scripture teaches that circumcision is in that place that distinguishes between male and female.

וערל זכר: כאן למד שהמילה באותו מקום שהוא ניכר בין זכר לנקבה:

who will not circumcise: When he reaches the age when he becomes liable for punishment, then [his soul] will be cut off (Shab. 133b), but his father [who does not circumcise him] is not punishable by “kareth” (spiritual excision), but is guilty of transgressing a positive commandment (Yeb. 70b).

and I will bless her: with breast feeding, when she required it, on the day of Isaac’s feast, for people were murmuring against them, that they had brought a foundling from the street and were saying, “He is our son.” So each one brought her child with her, but not her wet nurse, and she (Sarah) nursed them all. That is what is said (below 21:7): “Sarah has nursed children.” Gen. Rabbah (53:9) alludes slightly to this. — [from B.M. 87a]

And Abraham fell on his face and rejoiced: Heb. וַיִּצְחָק. Onkelos renders this as an expression of joy, וַחֲדִי “and he rejoiced,” but the one [וַתִּצְחָק] in the case of Sarah (below 18:12) [he renders] as an expression of laughter. You learn that Abraham believed and rejoiced, but Sarah did not believe and ridiculed, and for this reason, the Holy One, blessed be He, was angry with Sarah, but was not angry with Abraham.

Will [a child] be born to on, etc.: There are questions which are positive assertions, like (I Sam. 2:27): הֲנִגְלֹה נִגְלֵיתִי, “Did I appear?” [meaning: “of course I appeared!”]; (II Sam. 15:27): הֲרֹאֶה אַתָּה, “Do you see?” [meaning: “of course you see!”] This too is a positive assertion, and so did he say to himself, “Was such kindness done to anyone else, that the Holy One, blessed be He, is doing for me?”

and will Sarah, who is ninety years old: Shall she be worthy of giving birth? Now although the first generations begot children at the age of five hundred, in Abraham’s time, the years were already lessened, and weakness had come to the world. Go out and learn this from the ten generations from Noah to Abraham, who hastened to beget children at the age of sixty and seventy.

and you shall name him Isaac: Heb. יִצְחָק, because of the rejoicing (צְחוֹק) (Mid. Chaseroth v’Yetheroth). And some say: because of the ten (י) trials, and Sarah’s ninety (צ) years, and the eighth (ח) day on which he was circumcised, and Abraham’s hundred (ק) years. (Pirkei d’Rabbi Eliezer , ch. 32). (Other editions: “And My covenant.” Why is this written? Is it not already written (verse 9): “And you shall keep My covenant, you and your seed, etc.?” But because He said (verse 7): “And I will establish, etc.,” one might think that the sons of Ishmael and the sons of Keturah are included in the establishment [of the covenant]. Therefore, Scripture states: “And I will establish My covenant with him,” and not with others. Now, why does it say [again in verse 21]: “But My covenant I will establish with Isaac?” This teaches us that he was holy from the womb. Another explanation [for the repetition of verse 19]: Said Rabbi Abba: Scripture here derives an a fortiori conclusion regarding the son of the mistress from [what is written regarding] the son of the handmaid. It is written here: “Behold I have blessed him, and I will make him fruitful, and I will multiply him.” This refers to Ishmael. How much more so, “But My covenant I will establish with Isaac!” (Gen. Rabbah 47:5).

My covenant: The covenant of circumcision shall be given over [only] to the seed of Isaac. See Sanh. 59.

את בריתי: ברית המילה תהא מסורה לזרעו של יצחק:

20And regarding Ishmael, I have heard you; behold I have blessed him, and I will make him fruitful, and I will multiply him exceedingly; he will beget twelve princes, and I will make him into a great nation.

22And He finished speaking with him, and God went up from above Abraham.

כבוַיְכַל לְדַבֵּר אִתּוֹ וַיַּעַל אֱלֹהִים מֵעַל אַבְרָהָם:

from above Abraham: This is a euphemism used in reference to the Shechinah, and we learn that the righteous are the chariot of the Omnipresent. — [from Gen. Rabbah 47:6, 82:6]

מעל אברהם: לשון נקיה הוא כלפי שכינה, ולמדנו שהצדיקים מרכבתו של מקום:

23And Abraham took Ishmael his son and all those born in his house and all those purchased with his money, every male of the people of Abraham's household, and he circumcised the flesh of their foreskin on that very day, as God had spoken with him.

on that very day: On the very day that he was commanded (Mid. Ps. 112:2), during the day and not at night. He was afraid neither of the heathens nor of the scorners. [He circumcised in the light of day] so that his enemies and his contemporaries would not say,“Had we seen him, we would not have allowed him to circumcise and to fulfill the commandment of the Omnipresent” (Gen. Rabbah 47:9).

when he was circumcised of the flesh of his foreskin: Concerning Abraham, it does not say אֵת, because he was lacking only the severing of the flesh, because it had already been flattened out by intercourse, but Ishmael, who was a youth, required that the foreskin be severed and the corona be uncovered. Therefore, in his case, it is אֵת. Gen. Rabbah (47:8).